Imaging systems with front side illuminated near infrared imaging pixels

ABSTRACT

An imaging system may include an image sensor having front side illuminated near infrared image sensor pixels. Each pixel may be formed in a graded epitaxial substrate layer such as a graded p-type epitaxial layer or a graded n-type epitaxial layer on a graded p-type epitaxial layer. Each pixel may be separated from an adjacent pixel by an isolation trench formed in the graded epitaxial layer. A deep p-well may be formed within each isolation trench. The isolation trenches and photodiodes for the pixels may be formed in the graded p-type epitaxial layer or the graded n-type epitaxial layer. The graded p-type epitaxial layer may have an increasing concentration of dopants that increases toward the backside of the image sensor. The graded n-type epitaxial layer may have an increasing concentration of dopants that increases toward the front side of the image sensor.

This application claims the benefit of provisional patent applicationNo. 61/703,680, filed Sep. 20, 2012, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

This relates generally to imaging systems, and more particularly, toimaging systems with front side illuminated near infrared image pixels.

Modern electronic devices such a cellular telephones, cameras, andcomputers often use digital image sensors. Imagers (i.e., image sensors)often include a two-dimensional array of image sensing pixels. Eachpixel typically includes a photosensor such as a photodiode thatreceives incident photons (light) and converts the photons intoelectrical signals.

In some situations, it is desirable to capture images using infraredlight in addition to, or separately from, images captured using visiblelight. However, typical image pixels that are formed in a siliconsubstrate can have limited infrared imaging capability due to therelatively low absorption of near-infrared (NIR) light in silicon.Additionally, NIR photons penetrate deeper into a silicon substrate andcan generate pixel crosstalk which results in lower image sharpness inexisting sensors in response to NIR light.

It would therefore be desirable to be able to provide improved imagingsystems for capturing infrared images.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative imaging system that may include acamera module having an image sensor with front side illuminatedinfrared image pixels in accordance with an embodiment of the presentinvention.

FIG. 2 is a cross-sectional side view of illustrative front sideilluminated infrared image pixels that are formed in a graded p-typeepitaxial substrate in accordance with an embodiment of the presentinvention.

FIG. 3 is a cross-sectional side view of an illustrative front sideilluminated infrared image pixels that are formed in a combined gradedn-type and graded p-type epitaxial substrate in accordance with anembodiment of the present invention.

FIG. 4 is a flow diagram showing illustrative steps involved in formingfront side illuminated infrared image pixels in a graded p-typeepitaxial substrate in accordance with an embodiment of the presentinvention.

FIG. 5 is a flow diagram showing illustrative steps involved in formingfront side illuminated infrared image pixels in a combined graded n-typeand graded p-type epitaxial substrate in accordance with an embodimentof the present invention.

FIG. 6 is a block diagram of an imager employing the embodiments ofFIGS. 1-5 in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Electronic devices such as digital cameras, computers, cellulartelephones, and other electronic devices include image sensors thatgather incoming image light to capture an image. The image sensors mayinclude arrays of imaging pixels. The pixels in the image sensors mayinclude photosensitive elements such as photodiodes that convert theincoming image light into image signals. Image sensors may have anynumber of pixels (e.g., hundreds or thousands or more). A typical imagesensor may, for example, have hundreds of thousands or millions ofpixels (e.g., megapixels). Image sensors may include control circuitrysuch as circuitry for operating the imaging pixels and readout circuitryfor reading out image signals corresponding to the electric chargegenerated by the photosensitive elements.

An image sensor may include imaging pixels configured to respond tovarious colors of light. As examples, an image sensor may include redimage pixels, blue image pixels, clear image pixels, green image pixels,yellow image pixels, and/or infrared image pixels such as near infraredimage pixels. Near infrared image pixels may include an infrared colorfilter element that blocks or absorbs visible light while passing nearinfrared light onto photosensitive regions of the near infrared pixel.The image pixels in the image sensor may be front side illuminated (FSI)image pixels.

FIG. 1 is a diagram of an illustrative imaging system that uses an imagesensor having front side illuminated near infrared image pixels tocapture images. Imaging system 10 of FIG. 1 may be a portable electronicdevice such as a camera, a cellular telephone, a video camera, or otherimaging device that captures digital image data. Camera module 12 may beused to convert incoming light into digital image data. Camera module 12may include one or more lenses 14 and one or more corresponding imagesensors 16. Image sensor 16 may be an image sensor integrated circuitdie with an array of image pixels 30. Image pixels 30 may include one ormore front side illuminated (FSI) infrared image sensor pixels(sometimes referred to herein as front side illuminated near infraredimage pixels, front side illuminated image pixels, image pixels, orpixels). Image sensor 16 may include one or more arrays of image pixels30 such as red image pixels, blue image pixels, clear image pixels,green image pixels, yellow image pixels, and/or near infrared imagepixels.

During image capture operations, light from a scene may be focused ontoan image pixel array on image sensor 16 by lens 14. Image sensor 16provides corresponding digital image data to control circuitry such asprocessing circuitry 18.

Circuitry 18 may include one or more integrated circuits (e.g., imageprocessing circuits, microprocessors, storage devices such asrandom-access memory and non-volatile memory, etc.) and may beimplemented using components that are separate from camera module 12and/or that form part of camera module 12 (e.g., circuits that form partof an integrated circuit that includes image sensors 16 or an integratedcircuit within module 12 that is associated with image sensors 16).Image data that has been captured by camera module 12 may be furtherprocessed and/or stored using processing circuitry 18. Processed imagedata may, if desired, be provided to external equipment (e.g., acomputer or other device) using wired and/or wireless communicationspaths coupled to processing circuitry 18. Processing circuitry 18 may beused in controlling the operation of image sensors 16.

FIG. 2 is a cross-sectional side view of a portion of image sensor 16that includes front side illumination infrared image sensor pixels 30.As shown in FIG. 2, each image pixel 30 may include a photosensitiveelement such as photodiodes 38 formed in a substrate such as substratelayer 32 (e.g., an active p-type epitaxial substrate). Substrate layer32 may be a graded p-type epitaxial layer (sometimes referred to hereinas a graded p-epi substrate) in which the concentration of p-typedopants increases toward the backside of sensor 16 in the direction ofarrow 34.

In order to provide effective NIR photon generated charge collection andtransfer of the generated charges to the surface of photodiodes (PDs) 38and effective suppression of optical and electrical pixel crosstalk,graded p-epi substrate 32 may include deep trench pixel isolation usingtrenches 48 between pixels and additional deep implantation of p-wells49 through trenches 48.

Each pixel 30 may include a photodiode 38 formed in substrate 32, acolor filter element 36 and a microlens 39. Each microlens 39 may focusimage light such as NIR light 37 through an associated color filterelement 36 and onto the photodiode 38 of that pixel. Optical andelectrical crosstalk may be prevented by pixel isolation structures suchas pixel isolation structures 46 that separate the photodiodes ofadjacent pixels.

Each pixel isolation structure 46 may be formed from a deep trench 48 insubstrate 32 and deep p-well 49 formed through that trench. Formation ofp-wells 49 may also form a passivation layer on the interior surfaces oftrenches 48. Each trench 48 may be filled with a material such assilicon oxide. Trenches 48 may be formed in a very deep trench isolation(VDTI) process. The VDTI process may be followed by a very deep p-wellimplantation process via the deep trench that effectively isolatesphotodiodes 38 and pixels 30 from each other. Additionally, VDTItrenches 48 may provide optical isolation between adjacent pixels 30 tothe depth of VDTI trenches 48. VDTI trenches may be completely filledwith material such as silicon oxide or, if desired, some air may be leftin the VDTI trench between sidewalls of the trench to further opticallyisolate pixels 30 from each other.

As shown in FIG. 2, dielectric stack 40 may be formed on the frontsurface of substrate 32. Dielectric stack 40 may be interposed betweenphotodiodes 38 and microlenses 39 so that light such as light 37 isfocused through dielectric stack 40 onto photodiodes 38 by microlenses39. Dielectric stack 40 may have alternating layers of metal 44 anddielectric material 42. Metal interconnects 44 may be patterned metallayers within in dielectric stack 40. Metal interconnects 44 may beformed from a suitable metal such as copper or aluminum. Dielectricstack 40 may include metal vias. Dielectric stack may have, for example,two or more metal layers, four or more metal layers, six or more metallayers, or other suitable numbers of metal layers. Dielectric stack 40may also be known as interlayer dielectric (ILD). Metal layer and vialayers may be known as interconnect layers.

In a configuration of the type shown in FIG. 2, photodiodes 38 mayinclude a p-n junction at a depth of between 3 microns and 4 microns(for example) from the front side of substrate 32. However, thisconfiguration is merely illustrative. If desired, the depth of the p-njunction and the resulting NIR imaging efficiency may be furtherincreased using a substrate having a combination of graded p-typeepitaxial (p-epi) and graded n-type epitaxial (n-epi) layers. This typeof configuration is shown in FIG. 3.

As shown in FIG. 3, an n-type epitaxial substrate layer such as gradedn-epi layer 50 may be formed on graded p-epi layer 32 so that n-epilayer 50 is interposed between p-epi layer 32 and dielectric stack 40.Photodiodes 38 may be formed in n-epi layer 50. The concentration ofp-epi carriers may increase toward the backside of image sensor 16 inthe direction of arrow 34 and the concentration of n-epi carriers mayincrease toward the frontside of image sensor 16 in the direction ofarrow 52.

In this way, image sensor 16 may be configured so that the p-epi carrierconcentration in substrate 32 is highest at the bottom, less in themiddle, and completely eliminated in n-epi portion 50 and the n-episubstrate 50 has a concentration of n-type carriers that increases inthe direction of the silicon surface. Additional implantation of PDs 38with the highest n-type concentration creates vertical electrical fieldsin the image sensor, thereby effectively pulling photon generatedelectrons to the silicon surface for easy transfer of the charge withinthe pixel. This arrangement allows formation of a p-n junction down tobetween 8 microns to 10 microns in depth (for example) using existingimplantation tool capabilities. This type of arrangement may extend NIRphoton absorption in the substrate to a depth of up to 12 microns to 16microns with improved pixel crosstalk control.

The configurations described above in which photon-generated electronsaccumulate in photodiodes formed in graded n-epi and p-epi substrateswith deep trench and deep p-well isolation between the photodiodes aremerely illustrative. If desired, pixels 30 may collect photon-generatedholes in photodiodes formed in graded p-epi substrates in which theconcentration of p-type dopants increases toward the surface of thesilicon and in which deep trench and deep n-well isolation structuresare formed between the photodiodes. In another example, hole collectingphotodiodes with very deep p-n junctions may be formed in a graded p-episubstrate that is formed on a top of a graded n-epi substrate in whichthe concentration of p-type dopants in the p-epi substrate increasestoward the surface of the silicon and the concentration of n-typedopants in the graded n-epi substrate increases toward the backside ofthe silicon and in which deep trench and deep n-well isolationstructures between the photodiodes extend up to or beyond the interfacebetween the graded p-epi and the n-epi substrates.

FIG. 4 is a flow diagram showing illustrative steps that may be used inthe formation of an image sensor of the type shown in FIG. 2. As shownin FIG. 4 a substrate such as a silicon substrate having at least aportion with a graded p-epi layer 32 may be provided to pixel processingequipment 60. Pixel processing equipment 60 (e.g., deposition equipment,patterning equipment, implantation equipment, annealing equipment, orother suitable equipment for forming pixel components such asphotodiodes and transistors in a silicon substrate) may be used to formpixel circuitry 38P (e.g., a blanket photodiode) in substrate 32.Blanket photodiode 38P may be formed in part by forming a p-n junctionin substrate 32 by implanting a high concentration of n-type dopants asdeep as possible in substrate 32.

Trench formation equipment 62 (e.g., masking equipment, etchingequipment, etc.) may be used to perform very deep trench isolation(VDTI) operations to form trenches 48 in substrate 32 in which blanketphotodiode 38P has been formed, thereby isolating photodiodes 38 of eachindividual pixel.

Without filling trenches 48, implantation equipment 64 may be used toimplant very deep p-wells 49 in trenches 48. Deep p-well implantationoperations may also passivate the sidewalls of trenches 48.

Sensor processing equipment 66 (e.g., deposition and patterningequipment, equipment for formation of metal layers, color filter layers,and microlens layers, trench filling equipment, etc.) may be used tofill trenches 48 with a filler material such as an oxide material (e.g.,by filling trenches 48 with an oxide material such as silicon oxide inthe presence of hydrogen and deuterium), to form pixel gates anddielectric stack 40 over photodiodes 38, and to form color filterelements 36 and/or microlenses 39 on dielectric stack 40 to form imagesensor 16.

FIG. 5 is a flow diagram showing illustrative steps that may be used inthe formation of an image sensor of the type shown in FIG. 3. As shownin FIG. 5 a substrate such as a silicon substrate having at least aportion with a graded p-epi layer 32 and a graded n-epi layer 50 may beprovided to pixel processing equipment 60. Pixel processing equipmentmay be used to form pixel circuitry 38P (e.g., a blanket photodiode) ingraded n-epi substrate layer 50. Blanket photodiode 38P may be formed inpart by implanting a high concentration of n-type dopants as deep aspossible in graded n-epi substrate layer 50.

Trench formation equipment 62 (e.g., masking equipment, etchingequipment, etc.) may be used to perform very deep trench isolation(VDTI) operations to form trenches 48 in substrate layer 50 in whichblanket photodiode 38P has been formed, thereby isolating photodiodes 38of each individual pixel.

Without filling trenches 48, implantation equipment 64 may be used toimplant very deep p-wells 49 in trenches 48 in layer 50. Deep p-wellimplantation operations may also passivate the sidewalls of trenches 48.P-wells 49 may extend to the interface between p-epi layer 32 and n-epilayer 50 or my extend partially into p-epi layer 32 from n-epi layer 50.

Sensor processing equipment 66 may be used to fill trenches 48 with afiller material such as an oxide material (e.g., by filling trenches 48with an oxide material such as silicon oxide in the presence of hydrogenand deuterium), to form pixel gates and dielectric stack 40 overphotodiodes 38 in layer 50, and to form color filter elements 36 and/ormicrolenses 39 on dielectric stack 40 to form image sensor 16.

FIG. 6 shows in simplified form a typical processor system 300, such asa digital camera, which includes an imaging device 200. Imaging device200 may include a pixel array 201 (e.g., an array of image sensor pixelssuch as front side illuminated near infrared pixels 30 of FIG. 2 or FIG.3). Processor system 300 is exemplary of a system having digitalcircuits that may include imaging device 200. Without being limiting,such a system may include a computer system, still or video camerasystem, scanner, machine vision, vehicle navigation, video phone,surveillance system, auto focus system, star tracker system, motiondetection system, image stabilization system, and other systemsemploying an imaging device.

Processor system 300, which may be a digital still or video camerasystem, may include a lens such as lens 396 for focusing an image onto apixel array such as pixel array 201 when shutter release button 397 ispressed. Processor system 300 may include a central processing unit suchas central processing unit (CPU) 395. CPU 395 may be a microprocessorthat controls camera functions and one or more image flow functions andcommunicates with one or more input/output (I/O) devices 391 over a bussuch as bus 393. Imaging device 200 may also communicate with CPU 395over bus 393. System 300 may include random access memory (RAM) 392 andremovable memory 394. Removable memory 394 may include flash memory thatcommunicates with CPU 395 over bus 393. Imaging device 200 may becombined with CPU 395, with or without memory storage, on a singleintegrated circuit or on a different chip. Although bus 393 isillustrated as a single bus, it may be one or more buses or bridges orother communication paths used to interconnect the system components.

Various embodiments have been described illustrating imaging systemshaving image sensors with arrays of front side illuminated (FSI) nearinfrared image sensor pixels. Each FSI near infrared image sensor pixelmay be formed in a graded epitaxial substrate layer such as a gradedp-type epitaxial substrate layer or a combined graded n-type epitaxialsubstrate layer on a graded p-type epitaxial substrate layer. Each frontside illuminated near infrared pixel may be separated from an adjacentfront side illuminated near infrared pixel by a deep isolation trenchformed in the graded epitaxial substrate layer. A deep p-well may beformed within each isolation trench (e.g., at the bottom of the trench).

The isolation trenches and photodiodes for the pixels may be formed inthe graded p-epi (graded p-type epitaxial) layer or the n-epi (gradedn-type epitaxial) layer. The graded p-epi layer may have an increasingconcentration of p-type carriers that increases toward the backside ofthe image sensor. The graded n-epi layer may have an increasingconcentration of n-type carriers that increases toward the front side ofthe image sensor.

The deep p-well in each trench may be formed by forming the trench inthe graded epitaxial substrate layer using very deep trench isolationtechniques and, before filling the trenches, implanting p-type dopantsinto each trench using deep p-well implantation techniques.

The foregoing is merely illustrative of the principles of this inventionwhich can be practiced in other embodiments.

What is claimed is:
 1. An image sensor, comprising: a graded p-typeepitaxial substrate layer; a plurality of front side illuminated imagesensor pixels formed in the graded p-type epitaxial substrate layer; anda plurality of isolation trenches in the graded p-type epitaxialsubstrate layer that separate adjacent front side illuminated imagesensor pixels.
 2. The image sensor defined in claim 1, furthercomprising: a deep p-well formed in each of the plurality of isolationtrenches.
 3. The image sensor defined in claim 2, further comprising: apassivation layer on sidewall surfaces of each of the plurality ofisolation trenches.
 4. The image sensor defined in claim 2, furthercomprising: a dielectric stack formed on the graded p-type epitaxialsubstrate layer.
 5. The image sensor defined in claim 4 wherein thedielectric stack comprises metal interconnects in a dielectric material.6. The image sensor defined in claim 5 wherein each front sideilluminated image sensor pixel includes a microlens.
 7. The image sensordefined in claim 6 wherein each front side illuminated image sensorpixel includes a photodiode formed in the graded p-type epitaxialsubstrate layer.
 8. The image sensor defined in claim 7 wherein thedielectric stack is interposed between the photodiodes and themicrolenses of the front side illuminated image sensor pixels.
 9. Animage sensor, comprising: a graded p-type epitaxial substrate layer; agraded n-type epitaxial substrate layer on the graded p-type epitaxialsubstrate layer; a plurality of photodiodes formed in the graded n-typeepitaxial substrate layer; and a plurality of isolation trenches in thegraded n-type epitaxial substrate layer that separate adjacentphotodiodes.
 10. The image sensor defined in claim 9, furthercomprising: a deep p-well formed in each of the plurality of isolationtrenches that extends from the graded n-type epitaxial substrate layerat least to the graded p-type epitaxial substrate layer.
 11. The imagesensor defined in claim 10, further comprising: oxide material in eachof the plurality of isolation trenches.
 12. The image sensor defined inclaim 11, further comprising: a dielectric stack on the graded n-typeepitaxial substrate layer.
 13. The image sensor defined in claim 12wherein the graded n-type epitaxial substrate layer is interposedbetween the dielectric stack and the graded p-type epitaxial substratelayer.
 14. The image sensor defined in claim 13, further comprising: aplurality of microlenses, wherein each of the plurality of microlensesis configured to focus light onto an associated one of the plurality ofphotodiodes through the dielectric stack.
 15. The image sensor definedin claim 14 wherein the dielectric stack in interposed between theplurality of microlenses and the plurality of photodiodes.
 16. The imagesensor defined in claim 15, further comprising: a plurality of colorfilter elements, wherein each of the plurality of microlenses isconfigured to focus light onto the associated one of the plurality ofphotodiodes through an associated one of the color filter elements andthrough the dielectric stack.
 17. A system, comprising: a centralprocessing unit; memory; input-output circuitry; and an imaging device,wherein the imaging device comprises: a graded epitaxial substratelayer; and an array of front side illuminated near infrared pixels inthe graded epitaxial substrate layer, wherein each front sideilluminated near infrared pixel is separated from an adjacent front sideilluminated near infrared pixel by a deep isolation trench formed in thegraded epitaxial substrate layer.
 18. The system defined in claim 17wherein the imaging device further comprises a deep p-well in each deepisolation trench.
 19. The system defined in claim 18 wherein the gradedepitaxial substrate layer comprises a graded p-type epitaxial layer. 20.The system defined in claim 19 wherein the graded epitaxial substratelayer further comprises a graded n-type epitaxial layer.